As is generally known in the art, secondary batteries are batteries that can be charged and discharged, unlike primary batteries which cannot be charged. Secondary batteries are widely used in cutting-edge electronic appliances, including cellular phones, laptop computers, and camcorders.
Lithium secondary batteries are examples of such secondary batteries and have operation voltages of about 3.7 V. These operation voltages are about three times greater than those of nickel-cadmium batteries or nickel-hydrogen batteries which are often used as power sources for many portable electronic appliances. Lithium secondary batteries also have high energy density per unit weight. For these reasons, lithium secondary batteries have been widely used.
Lithium secondary batteries generally use lithium-based oxides as the positive active materials, and use carbon materials as the negative active materials. Lithium secondary batteries are classified into liquid electrolyte batteries and polymer electrolyte batteries depending on the type of electrolyte used. Liquid electrolyte batteries are referred to as lithium ion batteries and polymer electrolyte batteries are referred to as lithium polymer batteries.
There are various types of lithium secondary batteries, including cylinders, cans, and pouches. As shown in FIGS. 1 and 2, a typical can-type lithium ion secondary battery includes a can 10, an electrode assembly 20 contained in the can 10, and a cap assembly 70 for sealing the top opening of the can 10. The can 10 may comprise a metallic member having the shape of a cuboid, and the can itself can be a terminal. The can 10 has an open top surface 10a, and the electrode assembly 20 is placed in the can 10 through the open top surface 10a. 
The electrode assembly 20 includes a positive electrode layer 30, a negative electrode layer 40, and a separator 50. The separator 50 is positioned between the positive and negative electrode layers 30 and 40, respectively, and the entire assembly is then wound, creating a jelly roll construction.
The positive electrode layer 30 includes a positive electrode collector 32 comprising thin aluminum foil and a positive electrode coating 34 comprising a lithium-based oxide as the main component. The positive electrode coating 34 is coated on both surfaces of the positive electrode collector 32. The positive electrode collector 32 also has non-coated areas 32a, which are not coated with the positive electrode coating 34. The non-coated areas 32a are located on both ends of the positive electrode layer 30. A positive electrode tab 36 is fixed to one of the non-coated areas 32a by ultrasonic welding such that both ends of the positive electrode tab 36 protrude from the upper end of the positive electrode collector 32. The positive electrode tab 36 usually comprises nickel or nickel alloy, but other metals may also be used.
The negative electrode layer 40 includes a negative electrode collector 42 comprising thin copper foil and a negative electrode coating 44 comprising a carbon material as the main component. The negative electrode coating 44 is coated on both surfaces of the negative electrode collector 42. The negative electrode collector 42 also has non-coated areas 42a, which are not coated with the negative electrode coating 44. The non-coated areas 42a are located on both ends of the negative electrode layer. A negative electrode tab 46 is fixed to one of the non-coated areas 42a by ultrasonic welding such that both ends of the tab 46 protrude from the upper end of the negative electrode collector 42. The negative electrode tab 46 usually comprises nickel or nickel alloy, but other metals may also be used.
The separator 50 is positioned between the positive and negative electrode layers 30 and 40, respectively, thereby insulating the electrode layers from each other. The separator 50 comprises polyethylene, polypropylene, or a composite film of polyethylene and polypropylene. The separator 50 usually has a width larger than the widths of the positive and negative electrode layers 30 and 40, respectively, to prevent a short circuit between the electrode layers.
The cap assembly 70 includes a cap plate 71, an insulation layer 72, a terminal plate 23, and a negative electrode terminal 74. The cap assembly 70 is first coupled to a separate insulation case 79, and is then coupled to the open top surface 10a of the can 10, thereby sealing the can.
Heat is generated in the can either when the battery is overcharged or overdischarged, or when a short circuit occurs between the electrodes. In particular, the heat concentrates on the part of the can having increased internal resistance, i.e. where different metals are bonded together to weld the electrode layer to the electrode tab. As heat concentrates around the electrode tab, the separator, which insulates the positive and negative electrode layers from each other, melts and contracts. As a result, an additional short circuit occurs between the electrode layers.
Secondary batteries tend to have larger capacity, thereby increasing the energy density of such batteries. The heat generated at the electrode tabs of these batteries due to initial heating causes short circuits between the electrode layers. As a result, overheating and explosion of these secondary batteries is more frequent.